The invention relates to pumps designed to inject fuel directly into the combustion chambers of a spark ignition engine. Unlike the diesel fuel used with diesel engines, the fuels used in spark ignition engines (gasoline and liquefied petroleum gas) do not lubricate the surfaces with which they are in contact. Piston-operated positive displacement pumps compress the fuel directly, incurring a risk of seizing. Furthermore, the only way of controlling the pressure of these pumps, the cylinder capacity of which is fixed, is by dissipating energy in a return path, which impairs the energy yield, heats the fuel and can give rise to cavitation.
In order to reduce the risk of seizing, a pump has already been proposed (FR-A-2 603 347) which has fuel compression chambers, each of which is bounded by a diaphragm separated by hydraulic fluid from a piston driven by a rotating plate. The stroke of the piston and the volume of liquid occupying the compartment defined by the piston and the diaphragm is constant, which does not resolve the problem of regulating the delivery rate and injection pressure.
An injection pump is also known (U.S. Pat. No. 5,520,523) which has a supply chamber for feeding an injection circuit, separated by a diaphragm from a compartment defined by a bore in the housing and by a piston arranged to be reciprocated during operation. The chamber is connected to a fuel supply and to the injection circuit by non-return valves. The pressure of the liquid occupying the compartment can be adjusted.